In general, previous instruments utilizing photoelectric techniques have been limited to monitoring blood flow and have not provided accurate blood flow rate measurements. Some of these previous techniques for blood flow rate monitoring have used a plethysmograph to monitor blood flow through an organ, typically the pinna of an ear or one of the patient's fingers. See, for example, an article in the American Journal of Physiology, 1940, Volume 130, No. 1 by Alrick B. Hertzman and John B. Dillon, entitled "Distinction Between Arterial, Venous and Flow Components in Photoelectric Plethysmography in Man." Also see, for example, U.S. Pat. No. 3,796,213 issued to Frederick Richard Neason Stephens on Mar. 12, 1974 and entitled, "Perfusion Monitor."
Photoelectric techniques for monitoring or measuring blood perfusion are based on the phenomena that changing blood volume gives rise to a changing light absorption and hence changes in the amplitude of a signal produced by a photosensitive device. Photoelectric plethysmographs are used extensively for studying relative changes in skin blood flow and and as a transducer for heartrate monitors. However, problems associated with calibration, linearity, and stability, have all but eliminated the use of photoelectric plethysmography for accurate non-invasive blood flow measurement.
Also, oximeters have been used to measure the quantity of oxygen in a patient's blood. However, in many instances the usefulness of the oximeter has been limited because certain patients, e.g., those in post-surgical recovery, have sufficient peripheral vaso-constriction to limit the application of the oximeter. This is because while the oximeter can indicate when the quantity of blood in a tissue being measured is sufficient it cannot give an indication of its flow state. Consequently, because of the insufficient blood flow, the oximeter typically analyzes a mixture of venous and arterial blood and thus gives an A.sub.os measurement of smaller value than it should be. It is therefore necessary to accurately measure the blood flow rate through the tissue to calibrate the oximeter readings, but, as stated above, prior art perfusion meters have not provided the desired accuracy and convenience. Hence, to determine the flow rate of blood within a tissue area under test and to provide a method for estimating the accuracy of an oximeter reading, the perfusion meter in accordance with the present invention has been developed.
The physiological parameter that makes possible all photoelectric plethysmographic techniques is the pulsatile color changes associated with blood flow through the microcirculatory vessels. As mentioned above, photoelectric plethysmographic techniques have been used for studying relative changes in skin blood flow but many problems are encountered when attempts are made to make accurate perfusion measurements. For example, a primary cause of linearity problems is the response characteristics of the photosensitive device used in the plethysmograph.
All known photosensitive devices, with the exception of a photodiode which has a low level output, have non-linear response characteristics similar to that shown in FIG. 1. Also photosensitive devices have some variation with parameters other than light but in general, the more sensitive the device the worse the stability problem. For example, photoconductive cells typically suffer from large variations with temperature and light history problems so that their usefulness has definitely been limited, at least as it applies to accurate blood flow rate measurement. Phototransistors, while being fairly stable, suffer from non-linearity problems as well as being susceptible to radio frequency interference because of their rectifying junctions. Hence, even with calibration, which was only temporary at the best, prior art photoelectric plethysmographic techniques (transmissive and reflective) could not reliably be used for accurate non-invasive blood flow rate measurement.
In accordance with the preferred embodiment of the present invention, a light emissive device provides illumination through body tissue to a photosensitive device. A signal is produced which represents the log of the transmitted light. Changes in the log of the signal produced in response to the transmitted light are utilized to produce indications representing changes in the pulsatile blood flow rate.